<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    /* let num = '20件', uname = '张三'
    let age
    age = 18
    age = 20
    alert(age)
    document.write(age)
    console.log(uname, num)
    // console.log(num)
    console.log(age) */

    // 交换变量
    let a = 100
    let b = 2000
    let temp = a
    a = b
    b = temp
    console.log(a, b)
  </script>
</body>

</html>